A semiconductor wafer is formed with a circuit and then bonded to an adhesive sheet, followed by being arranged to each step of cutting (dicing) into small element pieces, washing, drying, drawing (expanding) of the adhesive sheet, peeling (picking up) the small element pieces from the adhesive sheet, mounting, and the like. The adhesive sheet (dicing tape) used in these steps is desired to have sufficient tack strength for the cut small element pieces (chips) from the dicing step to the drying step while having reduced tack strength to cause no adhesive transfer in the picking up step.
Meanwhile, with the increase in performance and miniaturization of IC devices in recent years, semiconductor chips are more reduced in thickness. Wafers conventionally with a thickness of approximately 350 μm are expected to be thinned even to 100 μm or less.
Silicon used as a semiconductor wafer is, however, a brittle material and has a risk of breakage during transportation and processing when being thinned. In particular, cracking or chipping during dicing of such a wafer with a rotating blade significantly reduces transverse strength of the chips.
As a method of inhibiting chipping, a so-called stealth dicing wafer dividing method is proposed where laser light at a wavelength in an infrared region is focused inside a semiconductor wafer to form a modified layer and the wafer is divided starting from the modified layer (PTL 1).
In this system, laser is generally incident from the circuit surface side of the wafer. The laser light, however, does not pass through a circuit surface with a test element group (TEG) or a metal film and is not capable of forming a modified layer inside the wafer.
To solve this problem, a method of forming a modified layer inside a wafer is proposed where laser light is irradiated through an adhesive sheet bonded to fix the wafer for incidence of the laser light from the back surface side of the wafer.
PTL 2 proposes an adhesive sheet having a Young's modulus at 23° C. from 30 to 600 MPa, a linear transmittance at a wavelength of 1064 nm of 80% or more, and a phase difference at a wavelength of 1064 nm of 100 nm or less.
PTL 3 proposes an adhesive sheet having a parallel transmittance in a wavelength range from 400 to 1100 nm of 80% or more and arithmetic mean roughness Ra on a backside of a substrate film (surface opposite to the surface with an adhesive layer formed thereon) from 0.1 to 0.3 μm.